Obesity is a complex and chronic disease with many causes. It is not simply a result of overeating. Research has shown that genetics can play a significant role in determining a person's body weight, particularly for morbidly obese people. Diet and exercise may have a limited ability to provide effective, long-term relief for obese people.
Factors such as the environment, metabolism, eating disorders and certain medical conditions also may contribute to obesity.
Research has shown that a person's genes play an important role in their tendency to gain weight. Just as some genes determine eye color or height, others affect appetite, ability to feel full or satisfied, metabolism, fat-storing ability and even natural activity levels.
Environmental and genetic factors are closely intertwined. If you have a genetic predisposition towards obesity, the modern American lifestyle and environment may make controlling weight more difficult.
Fast food, long days sitting at a desk and suburban neighborhoods that require cars exacerbate hereditary factors such as metabolism and efficient fat storage. For those suffering from morbid obesity, anything less than a total change in environment usually results in failure to reach and maintain a healthy body weight.
We used to think that a person could lose weight if they burned more calories than they consumed. Now we know that for some people, it's not that simple.
Obesity researchers now refer to a theory called the "set point," a sort of thermostat in the brain that makes people resistant to either weight gain or loss. If you try to override the set point by drastically cutting your calorie intake, your brain responds by lowering metabolism and slowing activity. You then gain back any weight you lost.
Many obese and morbidly obese people suffer from eating disorders. In these cases, behavior and diet modification therapy are recommended to help treat the eating disorder before weight loss surgery is considered.
There also are certain medical conditions, such as hypothyroidism, that can cause weight gain and may be treated with medication.
Reviewed by health care specialists at UCSF Medical Center.